Singlet oxygen production following irradiation. Cultures were incubated with DADB (10 μM) + 0.5 μM BPD or 40 μM NPe6 for 60 min, and then refed with fresh medium and irradiated at 690 nm (BPD, 37.5 mJ/sq cm), or at 660 nm (NPe6, 75 mJ/sq cm), or sequentially at both wavelengths. Additional cultures were incubated with only DADB. Images of DADB fluorescence were captured in (A) untreated control cultures, or directly after: (B) low-dose BPD PDT, (C) low-dose NPe6 PDT, (D) sequential NPe6-BPD PDT. Panel (E) shows pixel brightness analyses of ~100 cells per field. Note that DADB fluorescence is lost upon interaction with singlet oxygen. *Significantly less than either A or C (p<0.05). There was no significant difference between A and C, or B and D.